A yellow buoy may mark which of the following areas?

A yellow buoy is specifically used as a navigational aid to indicate special warning areas that do not fit into the standard categories marked by other colored buoys. When it comes to designated navigation marks, a yellow buoy is mainly employed for alerting mariners to areas that may require special caution, such as anchorage areas, which may be less safe or serve specific purposes where vessels are allowed to anchor under certain conditions.

Anchorage areas are often marked to guide vessels to suitable locations for securing them temporarily, and a yellow buoy denotes these spots. The color specifically signals that there may be conditions or regulations that mariners should adhere to while transiting or anchoring in those areas.

The other options listed do not typically use yellow buoys. For instance, wrecks and shoal areas are represented by specific marks that reflect hazard or caution in more traditional signal colors. Likewise, middle grounds are usually marked to indicate safe passage or navigable waters, and not with yellow buoys. Thus, identifying an anchorage area with a yellow buoy aligns with navigational standards and best practices established by the Coast Guard.